This invention relates to extruder screws for melting, mixing and extruding semi-crystalline polymers at high throughput rates, and particularly to high output extruder screws having a transition section including a series of alternating compression and relief zones with each compression zone having a decreasing channel depth and each relief zone having an increasing channel depth.
Conventional extruders typically have smooth cylindrical barrels fitted with extruder screws having helical channels such that rotation of the screw will advance a granular or powdered plastic toward the discharge end of the extruder. Melting of the plastic is promoted by maintaining the barrel at an elevated temperature, but also, and more effectively, by the mechanical energy provided by rotation of the screw. The efficiency of the melting and conveying or pumping of the plastic is influenced by the design of the extruder screw. The nature of the plastic being extruded also can substantially influence the rate of extrusion, temperature of the extrudate, uniformity of melting, and other parameters.
Most conventional extruder screws include a feed section having a deep constant depth helical channel for the conveying of unmelted solids, a compression or transition section of uniformly decreasing channel depth to assist in melting, and a meter section of constant reduced channel depth to complete polymer melting and control throughput. The extrusion characteristics of an extruder screw can be altered by the selection of the depths of the feed and meter sections and the relative lengths of the feed, transition and meter sections. It has been known that to obtain higher outputs the approach is to use screws having deeper channel depths or use higher screw speeds. However, deeper channel depths decrease melting efficiency and also cause temperature variations resulting from unmelted polymer which limits productivity. The use of higher screw speeds causes undesirably high extrudate temperatures. Conventional extruder screws have limited melting and conveying capacity and when attempting to obtain high throughput rates, melting is often inadequate to obtain a completely melted extrudate. Inadequate melting in the transition section can result in plugging, which is the filling up of the channel with solid materials. Plugging is believed to be a major cause of undesirable surging or pressure fluctuations. It is known that plugging can be avoided by the use of long feed and transition sections or by the use of lower compression ratio screws. The compression ratio is the ratio of the feed channel depth to the meter channel depth. Any of these changes, however, tend to reduce the melting capacity of a screw design.
To improve melting, some extruder screws have been made with mixing devices such as circumferential rows of pins or longitudinal fluted sections with increased flight clearance to pass the melt and trap unmelted material. Such screws, however, are not capable of handling large quantities of unmelted material and thus the mixing devices are located in the meter section, and most desirably, near the end of the meter section close to the discharge end. Consequently, such screws with mixing devices do not promote melting of the material in the transition section.
Other conventional extruder screws used to promote melting are multi-stage screws where the channel depth varies in a cyclic manner in the direction of flow as shown, for example, in U.S. Pat. No. 3,023,456. In such multi-stage screws, there are several feed sections, several transition sections, one or more relief sections and several meter sections. The difficulty with such extruder screws is that the first feed and transition section must be short to permit the use of succeeding ones. The use of such short feed and transition sections causes plugging at high throughput rates. Also, such screws have zones of zero pressure which require regions of high shear to reduce the pressure. This results in high plastic melt temperatures.
Another conventional type of screw using relief zones is the wave screw which has sinusoidal changes in channel depth in only the meter section. This screw uses a conventional feed and transition section and encounters the plugging problems of other conventional screws.
Another conventional type of screw used to improve melting is the melt-drain screw which either uses a double flight to segregate the melt from the unmelted solid or drains the melt into passages within the core of the screw. Such melt-drain screws are expensive to fabricate.